Molecular weight recognition in the multiple-stranded helix of a synthetic polymer without specific monomer-monomer interaction.

Stereoregular isotactic and syndiotactic poly(methyl methacrylate)s (it- and st-PMMAs) are known to form a multiple-stranded complementary helix, so-called stereocomplex (SC) through van der Waals interactions, which is a rare example of helical supramolecular structures formed by a commodity polymer. In this study, we prepared SCs by using uniform it- and st-PMMAs and those with a narrow molecular weight distribution having different molecular weights and investigated their structures in detail using high-resolution atomic force microscopy as a function of the molecular weight and molecular weight distribution of the component PMMAs. We found that complementary it- and st-PMMAs with the longer molecular length determine the total length of the SC, and molecules of the shorter component associate until they fill up or cover the longer component. These observations support a supramolecular triple-stranded helical structure of the SCs composed of a double-stranded helix of two intertwined it-PMMA chains included in a single helix of st-PMMA, and this triple-stranded helix model of the SCs appears to be applicable to the it- and st-PMMAs having a wide range of molecular weights we employed in this study. In homogeneous double-stranded helices of it-PMMA, it has been found that, in mixtures of two it-PMMAs with different molecular weights, chains of the same molecular weight selectively form a double-stranded it-PMMA helix, or recognize the molecular weights of each other ("molecular sorting"). We thus demonstrate that molecular weight recognition is possible, without any specific interaction between monomer units, through the formation of a topological multiple-stranded helical structure based upon van der Waals interaction.

Study on blood compatibility with poly(2-methoxyethylacrylate)--relationship between surface structure, water structure, and platelet compatibility in 2-methoxyethylacrylate/2-hydroxyethylmethacrylate diblock copolymer.

Diblock copolymers composed of 2-methoxyethylacrylate (MEA) and 2-hydroxyethylmethacrylate (HEMA) were firstly prepared (the composition ratio = 90/10, 79/21, 66/34, and 48/52 mol/mol) by anion living polymerization. ESCA analysis of their surface structures (dry state) revealed that PMEA segment was segregated to the top surface in all of the polymers, whereas the results of contact angle of water (wet state) showed that the surfaces were covered with PHEMA segment. In vitro platelet adhesion test showed that these polymers had the excellent compatibility with platelet compared to PHEMA homopolymer. Water structure in the hydrated copolymers was investigated by DSC and freezing bound water was observed for all the polymers like PMEA homopolymer, whereas it was not found in PHEMA homopolymer. Further investigation of water structure based on the results of DSC and EWCMS (equilibrium water content by moisture sorption) suggested that freezing bound water existed in PHEMA segment in addition to PMEA segment. We have proposed that the water plays a key role in the appearance of good blood compatibility of the copolymer, according to our previous works (Tanaka et al. Biomacromolecules 2002;3:36-41, Tanaka et al. J Biomed Mater Res A 2004;68:684-695).

Structural characterization of alpha-terminal group of natural rubber. 1. Decomposition of branch-points by lipase and phosphatase treatments.

Deproteinized natural rubber latex (DPNR-latex) was treated with lipase and phosphatase in order to analyze the structure of the chain-end group (alpha-terminal). The enzymatic treatment decreased the content of long-chain fatty acid ester groups in DPNR from about 6 to 2 mol per rubber molecule. The molecular weight and intrinsic viscosity were reduced to about one-third after treatment with lipase and phosphatase. The Huggins' k' constant of the enzyme-treated DPNR showed the formation of linear rubber molecules. The molecular weight distribution of DPNR changed apparently after treatment with lipase and phosphatase. (1)H NMR spectrum of rubber obtained from DPNR-latex showed small signals due to monophosphate, di-phosphate and phospholipids at the alpha-terminus. Treatment of DPNR-latex with lipase and phosphatase decreased the relative intensity of the (1)H NMR signals corresponding to phospholipids, whereas no change was observed for the signals due to mono- and diphosphates. The residual mono- and diphosphate signals as well as some phospholipid signals after lipase and phosphatase treatments indicate that mono- and diphosphate groups are directly linked at the alpha-terminus with the modified structure, expected by aggregation or linking with phospholipid molecules.

Structural characterization of alpha-terminal group of natural rubber. 2. Decomposition of branch-points by phospholipase and chemical treatments.

The treatment of deproteinized natural rubber (DPNR) latex with phospholipases A(2), B, C, and D decreased significantly the long-chain fatty acid ester contents in DPNR and also the molecular weight and Higgins' k' constant, except for phospholipase D treatment. This indicates the presence of phospholipid molecules in NR, which combine rubber molecules together. Transesterification of DPNR resulted in the decomposition of the functional group at the terminal chain-end (alpha-terminal), including phospholipids and formed linear rubber molecules. The addition of small amounts of ethanol into the DPNR solution reduced the molecular weight and shifted the molecular weight distribution (MWD) comparable to that of transesterified DPNR (TE-DPNR). The addition of diammonium hydrogen phosphate into DPNR-latex in order to remove Mg2+ ions yielded a slight decrease in molecular weight and a slight shift in MWD. The phospholipids are expected to link with mono- and diphosphate groups at the alpha-terminal by hydrogen bonding and/or ionic linkages. The decrease in the molecular weight and Huggins' k' constant of DPNR demonstrates the formation of linear molecules after decomposition of branch-points by this treatment, showing that phospholipids participate in the branching formation of NR. The branch-points formed at the alpha-terminus are postulated to originate predominantly by the association of phospholipids via micelle formation of long-chain fatty acid esters and hydrogen bonding between polar headgroups of phospholipids.

Temperature gradient interaction chromatography and MALDI-TOF mass spectrometry analysis of stereoregular poly(ethyl methacrylate)s.

Temperature gradient interaction chromatography (TGIC) was applied for the separation of stereoregular poly(ethyl methacrylate) (PEMA) according to the tacticity. The three PEMA samples with differing tacticity (rr triad content 0, 53, and 91%) prepared by anionic polymerization were used. C18 bonded silica and a mixture of CH2Cl2 and CH3CN (30/70, v/v) were used as stationary and mobile phase, respectively. TGIC was able to separate the PEMA samples, showing the increasing retention in the order of decreasing rr triad contents; however TGIC elution peaks of the three PEMAs were not fully resolved but, rather, were partially overlapped. To isolate the tacticity effect from the molecular weight effect on the TGIC retention, the PEMA samples were fractionated by TGIC, and the accurate molecular weight of the fractions was determined by MALDI-TOF mass spectrometry. The fractions showed a much narrower molecular weight distribution than the mother PEMAs. The TGIC fractions of similar molecular weight but with different tacticity were fully resolved by TGIC, but mother PEMAs were not. These results indicate that the retention in TGIC is affected by both tacticity and molecular weight.